Fourier transform infrared (FTIR) spectrometers are utilized in the analysis of chemical compounds. In these instruments, a beam of infrared radiation having a band of infrared wavelengths is passed into an interferometer, typically a Michelson interferometer, and is modulated before being passed through the compound to be analyzed and then to a detector. The interferometer modulates the radiation received by it to provide an output beam in which many narrow ranges of infrared wavelengths are typically reduced or enhanced in intensity, with the affected range of wavelengths changing periodically over time. The time correlated output signal from the detector is analyzed by Fourier transformation to derive information on the characteristics of the sample through which the beam was passed.
In the typical Michelson interferometer used in such FTIR spectrometers, the input beam is received by a beamsplitter which partially passes the beam through to a moving mirror and partially reflects the beam to a fixed mirror, or vice versa, and the reflected beams are recombined at the beamsplitter to yield the output beam. The relative position of the moving mirror with respect to the beamsplitter and fixed mirror will determine which wavelengths constructively and destructively interfere when the beams from the two mirrors are recombined at the beamsplitter. The movement of the moving mirror toward and away from the beamsplitter results in the scanning of the constructively and destructively interfering wavelengths across a desired band of infrared wavelengths. Examples of such Michelson interferometer systems in FTIR instruments are shown in U.S. Pat. Nos. 4,799,001 and 4,847,878 and published Patent Cooperation Treaty application WO 93/14373.
It is critical in the design of FTIR instruments that the surfaces of the fixed mirror and the moving mirror be accurately held orthogonal to each other. Mirror position accuracy is crucial because deviations in the mirror alignment produce small errors in the time domain interferogram which may translate into large errors in the frequency domain spectrum. In a typical interferometer, mirror deviations larger than 1/10 wavelength of the received radiation beam are considered significant and can seriously degrade the quality of the instrument.
Static alignment of the mirrors of the interferometer is typically accomplished by means of precision fine pitch or differential screws at the back of the mirror which are manually adjusted to align the mirror to a desired position as perfectly as possible. This is a time consuming procedure requiring skill and experience, and adds to manufacturing expense and to field service costs because realignment in the field is often required.
Efforts have been made to eliminate the need to manually align the interferometer mirrors. Automatic static alignment at least relieves the user from performing time consuming realignments. For example, stepper motors have been used to carry out automatically the manual alignment procedure described above. Such devices typically use a digital computer which aids in the alignment of the static mirror at periodic service intervals. Disadvantages of this approach are the slow speed, large size, and high cost of, and the high precision bearings required for, the alignment mechanism.
Minor misalignments of the two mirrors can occur during operation due to tilting of the moving mirror as a result of bearing imperfections or vibrations transmitted to the mirror mechanically or acoustically from nearby machinery, fans and other active equipment. To attempt to adjust the moving or fixed mirror dynamically to compensate for the tilting of the moving mirror as it moves on its bearing requires more speed that can be readily obtained with a mechanism using lead screws and stepper motors. Another approach has been to use piezoelectric positioners to align dynamically the tilt of the mirrors. Such positioners are also typically large and expensive, and require high voltage (e.g., 1000 volts) drive levels. The power supplies required for such high voltages also create undesirable operating hazards as well as being relatively expensive. Approaches to dynamic mirror alignment using magnetic coils are shown in U.S. Pat. Nos. 5,239,361 and 5,276,545, although the size and weight of the components best adapts such systems to alignment of the fixed mirror rather than the moving mirror. While dynamic alignment of the fixed mirror can serve to optimize the spectral resolution of an FTIR spectrometer, in some cases dynamic alignment of the fixed mirror may move the output beam relative to the detector enough to reduce system stability. Thus, it would generally be advantageous to dynamically maintain proper alignment of the moving mirror despite the effects of bearing dynamics and vibrations. Alignment of the moving mirror using coils is shown in U.S. Pat. 4,480,914 but implemented using a complex structure that allows only a very limited range of axial movement of the moving mirror.